CURLOPT_SEEKFUNCTION(3)    curl_easy_setopt options    CURLOPT_SEEKFUNCTION(3)

NAME
       CURLOPT_SEEKFUNCTION - user callback for seeking in input stream

SYNOPSIS
       #include <curl/curl.h>

       /* These are the return codes for the seek callbacks */
       #define CURL_SEEKFUNC_OK       0
       #define CURL_SEEKFUNC_FAIL     1 /* fail the entire transfer */
       #define CURL_SEEKFUNC_CANTSEEK 2 /* tell libcurl seeking can't be done, so
                                           libcurl might try other means instead */

       int seek_callback(void *userp, curl_off_t offset, int origin);

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SEEKFUNCTION, seek_callback);

DESCRIPTION
       Pass a pointer to your callback function, which should match the proto-
       type shown above.

       This function gets called by libcurl to seek to a certain  position  in
       the  input  stream  and can be used to fast forward a file in a resumed
       upload (instead of reading all uploaded  bytes  with  the  normal  read
       function/callback).  It is also called to rewind a stream when data has
       already been sent to the server and needs to be sent  again.  This  may
       happen  when  doing a HTTP PUT or POST with a multi-pass authentication
       method, or when an existing HTTP connection is reused too late and  the
       server  closes the connection. The function shall work like fseek(3) or
       lseek(3) and it gets SEEK_SET, SEEK_CUR or  SEEK_END  as  argument  for
       origin, although libcurl currently only passes SEEK_SET.

       userp is the pointer you set with CURLOPT_SEEKDATA(3).

       The   callback   function  must  return  CURL_SEEKFUNC_OK  on  success,
       CURL_SEEKFUNC_FAIL to cause the upload operation to fail or  CURL_SEEK-
       FUNC_CANTSEEK  to  indicate that while the seek failed, libcurl is free
       to work around the problem if possible. The  latter  can  sometimes  be
       done by instead reading from the input or similar.

       If  you  forward  the input arguments directly to fseek(3) or lseek(3),
       note that the data type for offset is  not  the  same  as  defined  for
       curl_off_t on many systems!

DEFAULT
       By default, this is NULL and unused.

PROTOCOLS
       HTTP, FTP, SFTP

EXAMPLE
       TODO

AVAILABILITY
       Added in 7.18.0

RETURN VALUE
       Returns  CURLE_OK  if the option is supported, and CURLE_UNKNOWN_OPTION
       if not.

SEE ALSO
       CURLOPT_SEEKDATA(3), CURLOPT_IOCTLFUNCTION(3),

libcurl 7.54.0                 February 03, 2016       CURLOPT_SEEKFUNCTION(3)